The present invention relates to apparatuses for controlling pivoting of axles in industrial vehicles. More particularly, the present invention relates to an apparatus for controlling a pivotally supported axle and locking the pivoting of the axle with respect to a vehicle body when necessary.
An industrial vehicle such as a forklift typically has a rear axle that is supported pivotally with respect to its body. The axle pivots as the vehicle travels along bumpy roads to maintain traction between the vehicle wheels and the road surface. The pivotal axle also improves the riding comfort and the driving stability of the vehicle. However, the pivotal axle may reduce the driving stability of the vehicle when centrifugal force acting on the vehicle tilts the body as the vehicle changes directions.
Japanese Unexamined Patent Publication No. 58-211903 solves this problem by proposing a forklift that locks its axle to restrict pivoting of the axle with respect to the vehicle body when centrifugal force acting on the forklift exceeds a predetermined value. Accordingly, tilting of the vehicle body is restricted when a large centrifugal force acts on the forklift. Thus, the forklift changes directions in a stable state.
When an object carried by the forklift is lifted to a high position, the center of gravity of the forklift rises and destabilizes the forklift. Therefore, the axle may also be locked in such case when an object is lifted above a predetermined height. This would also stabilize the vehicle when lifting loads or when changing directions.
The axle is locked by a mechanism that includes a hydraulic damper, which is arranged between the body and the axle. The damper has two oil chambers connected to each other by a passage. Hydraulic oil flows between the oil chambers through the passage. The damper is locked when the movement of hydraulic oil between the oil chambers is restricted by closing the passage. When the flow of oil is prevented, the damper locks the axle and prohibits pivoting of the axle with respect to the body. When the passage is opened, movement of hydraulic oil between the oil chambers is allowed. This unlocks the damper and permits pivoting of the axle with respect to the body.
When the damper is unlocked, the hydraulic pressure in the oil chambers decreases immediately and sudden pivoting of the axle is permitted. Thus, a shock is often produced when the axle is unlocked. The shock is especially large if the axle is unlocked when the axle is pivoted at a large angle relative to the body when the vehicle is located on a level surface.
Japanese Unexamined Patent Publication No. 58-183307 describes an axle locking mechanism that prevents sudden pivoting of the axle when unlocking the axle. The mechanism has an absorber nipple, which includes a check valve and a fixed throttle valve. However, since two valves (the check valve and the throttle valve) are necessary, the structure of the axle locking mechanism is complicated. Furthermore, it is difficult to install the locking mechanism in the passage.
The arrangement of an electromagnetic proportional valve in the passage, which connects the two oil chambers of the damper, has also been proposed. When the proportional valve closes the passage, the damper is locked and pivoting of the axle is thus restricted. When the proportional valve is opened, the damper is unlocked and pivoting of the axle is permitted. The proportional valve adjusts the area opened in the passage to prevent sudden pivoting of the axle. However, the proportional valve is costly, adding significantly to the cost of the forklift.